The present invention generally relates to power-sense circuit for sensing the loss of back-up battery power when a main supply source is off. The present invention more particularly relates to such a power-sense circuit which is fully integrated and which, in addition to indicating the loss of back-up battery power, also provides an unambiguous indication that battery power was lost and that the main power source has not yet be reinstated.
Many systems require certain functions to remain operational when system power is turned off. These circuits require a battery back-up system and a circuit that detects if battery power is lost while the system main power source is off. One such application is in integrated circuits including central processing units. In such integrated circuits, real time clocks must be maintained by back-up battery power when the main power is off in order to avoid corruption of date and time information.
In the prior art, power-sense circuits have been traditionally implemented with discrete components including an external resistor and capacitor in series relation and external diodes for coupling the battery and the main power source to the integrated circuit. Such prior art implementations have also included a power-sense input to the integrated circuit which is coupled to the common junction of the external resistor and capacitor. When power is applied to such circuits, the voltage at the power-sense input will be delayed by the resistor and capacitor combination. This delay insures that the power-sense input will be low for some time after power is applied before it goes high. The integrated circuit samples the power-sense input and if the input is low while power is applied to the integrated circuit, the circuit sets a flag indicating that the integrated circuit had lost back-up battery power. Once the main power source is on and a battery is installed, the capacitor will store enough charge so that when the main power source is turned off, the battery will maintain the charge on the capacitor and also power the circuit. As a result, the power-sense input will remain high while the circuit is powered by the back-up battery.
While such implementations have been successful, they have required external components. Such external components require additional tooling to use them, thus increasing the cost of the system in which they are used. In addition, prior arrangements, in general, have required the entire integrated circuit to be back-up by a battery. As more functions are integrated onto a single integrated circuit, the lifetime of a battery decreases when the entire integrated circuit is battery backed-up.
The present invention overcomes the above disadvantages with prior art arrangements. By integrating the power-sense circuit, it is possible to back-up only a small portion of the integrated circuit, thus increasing the battery lifetime and reducing the tooling costs. These factors are particularly important with respect to large-scale integrated circuits.